The invention relates to a method of manufacturing a thread-reinforced, extruded rubber body by means of a shear head wherein the threads are continuously supplied, embedded in a rubber mixture which is in a continuous forward motion and fixed in the rubber body by vulcanizing the rubber mixture.
In the method of U.S. Pat. No. 4,671,761 the threads are embedded in the rubber mixture by means of a nozzle which is disposed downstream of the actual shear head. The individual parts of the nozzle are associated with each other in such way that they cannot be rotated. The threads are thus always parallel to each other and extend in direction towards the rubber body. In certain cases of application this is not fully satisfactory.
It is hence an object of the invention to further develop a method of this general kind such that the reinforcing threads are helically twisted in the finished rubber body.
In the method in accordance with the invention, the continuously fed threads are embedded in the continuously supplied rubber mixture which is instantaneously contained in the continuously rotating shear mandrel disposed in the shear head through orifices of this shear mandrel which are distributed in circumferential direction and extend through the front surface.
The speed of the forward motion of the rubber mixture is adjusted to the rotating speed of the shear mandrel in such a way that while the threads are embedded the rubber mixture approaches the lowest possible flow viscosity. The threads are thus on all sides exposed to the liquid medium which creates an excellent adhesive attaching in the final rubber body. In addition, they follow the rotating movement of the shear mandrel while being embedded in the rubber mixture. This causes the threads to be twisted around a common axis and helically extend through the final rubber body. The latter is thus reinforced not only in longitudinal direction but also additionally in circumferential direction, a fact which permits, for example, using the rubber body as ropes or the inside of ropes or as other form bodies which are exposed to a bending stress.
The threads can be made of any commercially available and used material, for example, natural, artificial or metallic textile threads. The use of mixtures thereof and embodiments consisting of several threads is also possible.
Inside the shear head the rubber mixture used is subject to a flexing caused by the relative twisting of the shear head in respect to the casing. When the rotating speed of the shear head increases, this flexing stress causes the temperature of the mixture used to rise. Hence, a heating up to vulcanization temperature is therefore possible without problems even in case of high rotational speeds. The vulcanization and the so caused hardening, however, do not automatically result when reaching the vulcanization temperature; this result is achieved only after a temporary, significant reduction of the flow viscosity of the mixture used during the beginning cross-linking reaction of the mixture which then causes the hardening. The different stages are clearly distinct from one another. This fact is used in accordance with the invention so as to embed the threads into the mixture at a time when the flow viscosity is as low as possible. This ensures a best possible incorporation of the threads in the rubber body so as to impart a best possible strength on the latter.
Advantageously, the retention time of the rubber mixture on the shear head is measured such that upon exiting through the orifice of the shear head the mixture is at least in an advanced stage of vulcanization. The position of the embedded threads can hence not be changed anymore. The possibly required vulcanization to completion can be carried out without problems in a tempering chamber.
Before introducing the threads into the shear head they can be twisted against the rotational sense of the shear mandrel. This is particularly advantageous when the threads are composed of different partial threads. The resulting reinforcement inside the finished rubber body is configured like a crosslaid rope. This imparts to the form body an excellent form resistance when the latter is exposed to tensile or bending stress.
While being introduced in the shear head, the threads can be subject to braking which can be carried out, for example, by means of a friction device. In this case, they are subject to an internal prestress inside the finished rubber bodies which further improves the dimensional stability thereof.
Through a concentric auxiliary opening which extends through the front side of the shear mandrel a preform can be continuously fed to the shear head. Generating an adhesive bonding in the aforesaid way this preform is jacketed by rubber bodies which are produced according to the aforesaid description. The number of layers successively disposed in radial direction can be freely selected and is determined only by the use of the rubber body.
In a correspondingly, in successive stages, and in a multiple, layers-produced rubber body, it is possible to change the rotational sense of the shear mandrel for each new layer. The layers of threads which successively follow one another in radial direction are associated with one another in a crosswise relationship. A dimensional stability is thus imparted to the rubber body.
In cases when the winding direction of the successive thread layers in the successive layers is the same, the finished rubber body has a comparatively greater elasticity. This makes it more suitable for cases when subject to shock stress during the intended use. Best possible properties are achieved in this case when the rubber mixture used simultaneously exhibits a high internal damping. This simultaneously suppresses vibrations, which can be caused in the rubber body by secondary effects when used in certain cases.
With regard to a strong mutual adhesion, it has proven itself to use like rubber mixtures for producing both the preform and the individual rubber layer. These mixtures are only prevulcanized before embedding the respective thread layer and the vulcanization is completed only after the final molding of the rubber body takes place. A tempering chamber can hence be used which reduces the occupation time of the shear mandrel and further improves the economical efficiency of the process in accordance with the invention. In addition, the layers successively disposed in radial direction are perfectly bonded to one another which, in turn, results in a correspondingly significant mechanical strength.
The rubber bodies obtained with the method in accordance with the invention can also be embedded in rubber mixtures before completing the vulcanization and be further manufactured to differently shaped form bodies; vulcanization can then be completed in a common process with the new bodies. With respect to the way of incorporating the reinforcing threads, the resulting reinforcing effect is significantly improved.
A particular advantage of the form parts obtained through the method in accordance with the invention is that the complete embedding of the reinforcing threads in the rubber mixture protects the threads from environmental effects in an excellent way. An exposure of the threads to corrosion is thus excluded as far as possible. Even steel wires can be used without problems for the manufacture of thread-reinforced, extruded rubber bodies which are employed in ropes used in a wet environment.